The present invention relates generally to substrate cleaning and, more particularly, to a wafer cleaning module and a method for cleaning a surface of a substrate.
As is well known to those skilled in the art, wafer preparation and cleaning operations are performed in the fabrication of semiconductor devices. One of the functions of a wafer cleaning operation is to remove contaminants such as adhered particles and adsorbed compounds, e.g., chemicals, from the surface of the wafer. For example, during a chemical mechanical planarization (CMP) operation designed to achieve global and local planarization of a wafer surface, abrasive particles in the slurry may bond to the wafer surface and chemicals in the slurry may be adsorbed over the wafer surface.
As the feature size of integrated circuits continues to decrease, it is imperative that wafer surfaces being subjected to fabrication operations be as close to 100% free of contamination as is practical. In one known cleaning operation, wafers are scrubbed with brushes in one or more brush scrubbing stations to remove contaminants from the wafer surface. During brush scrubbing, local jets created by the mechanical movements of the brushes produce sufficient shear forces to dislodge a high percentage of contaminants from the wafer surface. In some instances, the brush scrubbing operation is supplemented by megasonic agitation, which shakes the wafer surface and thereby assists in the breaking of the intermolecular bonding between contaminants and the wafer surface. In other instances, chemicals such as KOH and NH.sub.4 OH are introduced at the cleaning interface to enhance the cleaning process. When such chemicals are used, the distortion forces created by ions of the same electrical charge help overcome the intermolecular bonding between contaminants and the wafer surface.
The cleaning operations discussed above suffer from a number of shortcomings. First and foremost, these cleaning operations do not produce a contamination-free wafer because they do not remove about the last 1% of contaminants, which are bonded to the wafer surface with extremely strong intermolecular forces. Second, conventional brush scrubbing operations are coordinate centric. In other words, conventional brush stations do not scrub the center of wafers and the edges of wafers uniformly. Accordingly, multiple cleaning operations may be required to clean the entire surface of the wafer thoroughly. This is undesirable because it increases the overall time expended on wafer cleaning and thereby reduces throughput. Third, the brushes may cause micro-scratches on the wafer surface and thereby damage the wafer to such an extent that it must be discarded as scrap. This disadvantageously lowers the yield of the fabrication process. Fourth, the brushes may recontaminate the wafer surface by introducing residues of previously cleaned substrates to such a degree that multiple cleaning operations are required.
In view of the foregoing, there is a need for a method for cleaning a surface of a semiconductor wafer that provides contamination-free wafers without reducing throughput and without scratching or otherwise damaging the surface of the wafer.